Distribution, Abundance, and Population Trends of Bull Trout in Idaho

Abstract

Broad-scale declines in populations of bull trout Salvelinus confluentus over the past century or more led the Idaho Department of Fish and Game to implement statewide no-harvest regulations on bull trout in 1994 and ultimately led to a threatened listing under the U.S. Endangered Species Act in 1998. Despite this listing, quantitative evaluations of trends in abundance and estimates of population size over most of the species' historical range have not been made. We evaluated bull trout distribution, abundance, and trends in abundance using stratified sampling extrapolations of fish surveys (snorkeling and electrofishing) conducted at 2,521 survey sites (most distributed nonrandomly) across 77,447 km of stream. Bull trout were captured at 887 (35%) of the sites. Within the 262 local populations designated by the U.S. Fish and Wildlife Service within seven Idaho recovery units, the number of 70-mm total length and larger bull trout was estimated at 1.13 million; this estimate was most likely biased low due to sampling limitations. Long-term (>20 years) intrinsic rates of change (r) were negative for 10 of 16 bull trout populations up to 1994 (3 were significantly negative; 1 was significantly positive) and were positive for 14 of 17 populations after 1994 (1 was significantly negative; 5 were significantly positive). Over the entire period of record and all trend data sets, r averaged 0.01 ± 0.01 (mean ± 90% confidence interval), suggesting stability at a broad scale. During these same time periods, trends for other salmonids in much of the study area experienced similar declines through 1994 and increases after 1994, suggesting that environmental factors with influence over large geographical areas produced the recent positive trends. Once bull trout populations were detrended by use of linear regression residuals, there was little evidence of synchrony between populations. Our results suggest that despite declines from historical levels, bull trout in Idaho are presently widely distributed, relatively abundant, and apparently stable.